Je…desto/umso is one of several so-called compound conjunctions in the German language, which means that they are made up of two parts. What both parts have in common is that they feature the comparative form of an adjective, but whereas the je-clause is a subordinate clause in German that sends the conjugated verb to the end, the desto/umso-clause is a main clause in which the conjugated verb is second. Together, they are used to establish a causal connection between statements.
Since there is no difference between desto and umso, we can use the following formula to phrase sentences with je...desto or umso.
Je+ comparative+ subject+ conj. verb, desto/umso+ comparative+ conj. verb+subject
Je mehr Vokabeln man lernt, desto größer wird der eigene Wortschatz
(the more vocabulary you learn, the wider one’s vocabulary range becomes)
Je früher man einen Flug bucht, desto günstiger ist er normalerweise.
(the earlier you book a flight, the cheaper the price it normally is)
Je mehr sie ihr Deutsch üben, desto besser werden sie .
(the more they practice their German, the better they get).
One aspect that often strikes students as unusual is that the subordinate clause comes first. However, as you probably know from using other conjunctions, such as “als” and wenn”, it’s always possible to place the subordinate clause before the main clause for reasons of emphasis. Finally, according to German punctuation rules, a comma must separate the subordinate from the main clause.